<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let str = '我们好，你呢'
    let a = '了'

    function foo(str) {
      return str.indexOf(a)
    }

    console.log(foo(str))
  </script>
</body>
</html>
